How to add arguments to event handler in ES6 React when functions are bound in constructor

Question

With constructors in es6, we are advised to bind functions early, e.g.

class App extends React.Component {
  constructor(props) {
    super(props);
    this.handleClick = this.handleClick.bind(this); // bound early
  }

  handleClick() {
    // do stuff
  }
  ...
}

In ES5, we could typically call something like this.handleClick.bind(this, "foo") if we wanted to preserve context AND send an extra argument. What is the best pattern for this with the new class syntax in ES6 React?

For instance, if my class looked like the code below, how would I best access the "foo" and "bar" values? (I know the answer is not bind but this is how I could best illustrate the problem).

class App extends React.Component {
  constructor(props) {
    super(props);
    this.handleClick = this.handleClick.bind(this); // bound early
  }

  handleClick(event, value) {
    // do stuff with value ("foo" or "baz")
  }

  render() {
    return (
      <div>
        <button onClick={this.handleClick.bind("foo")} /> // incorrect code
        <button onClick={this.handleClick.bind("bar")} /> // incorrect code
      </div>
    )
  }
}

The first argument to .bind is always the this value. Therefore you would want this.handleClick.bind(null, "foo"). Since the function was already bound in the constructor, any value you pass as first argument is ignored. — Felix Kling
– Felix Kling, Commented Mar 2, 2016 at 5:22

Logan W · Accepted Answer · 2016-09-23 00:05:48Z

16

Think that:

onClick={this.handleClick.bind(this)}

Is the same as:

onClick={e => this.handleClick(e)}

So you can do:

    <button onClick={e => this.handleClick(e, 'foo')} />
    <button onClick={e => this.handleClick(e, 'baz')} />

In the end it is all just JavaScript.

edited Sep 23, 2016 at 0:05

Logan W

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answered Mar 1, 2016 at 21:40

elclanrs

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2 Comments

Duke Dougal Over a year ago

What exactly does "incorrect code" mean in this answer?

Jed Fox Over a year ago

This way will create a new function in each render, which slows down the app since React has to unregiste the old handler and register the new handler.

Bergi · Accepted Answer · 2016-03-01 22:01:34Z

6

In ES5, we could typically call something like this.handleClick.bind(this, "foo") if we wanted to preserve context AND send an extra argument.

You can do exactly the same in ES6 as well. It's not like bind was removed from the language :-)

class App extends React.Component {
  constructor(props) {
    super(props);
    this.handleFooClick = this.handleClick.bind(this, "foo"); // bind early
  }

  handleClick(value, event) {
    //        ^^^^^^ notice the bound values come first
    …
  }

  render() {
    return (
      <div>
        <button onClick={this.handleFooClick} /> // use early-bound
        <button onClick={this.handleClick.bind(this, "bar")} /> // bind late
        <button onClick={event => this.handleClick("foobar", event)} /> // arrow function
      </div>
    )
  }
}

edited Mar 1, 2016 at 22:01

answered Mar 1, 2016 at 21:47

Bergi

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4 Comments

Bergi Over a year ago

@elclanrs: Oops, that messed up arguments order in the bind call was a typo. this.handleClick.bind(this, "bar") should be equivalent to (...args) => this.handleClick("bar", ...args)

trevdev Over a year ago

This answer saved my bacon in a vanilla ES6/babel with classes scenario, thanks.

Jed Fox Over a year ago

Note that this will crate a new function each time the component is rendered, slowing down the app a bit.

Bergi Over a year ago

@JF Then use the early-bound alternative I've shown

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